The present invention relates to a magnetic resonance imaging system (MRI system). In particular, the present invention relates to data processing of spectroscopic imaging for measuring molecular distribution.
The magnetic resonance imaging system is a system which transmits a radio frequency electromagnetic field (RF) having a specific frequency to a measurement subject placed in a static magnetic field, thereby induces a magnetic resonance phenomenon, and acquires physical and chemical information of the measurement subject. Magnetic resonance imaging (MRI) that is widely spread at the present time is a method of mainly using the magnetic resonance phenomenon of the proton (1H) in the water molecule and imaging the difference in proton density and relaxation time which differ depending upon tissues. In addition, a method of measuring the map of each molecule containing not only the proton density but also the proton or phosphorus (31P), called spectroscopic imaging is also proposed. This is a method of separating magnetic resonance signals of each molecule on the basis of a difference of the magnetic resonance frequency (chemical shift) caused by a difference of molecular chemical bond, and imaging the density and relaxation time of each molecule. It is considered that this method makes possible a more precise diagnosis of diseases as compared with the case where the MRI is used. For example, it is considered that the diagnosis precision of cancers can be enhanced. In vigorously proliferating cancer tissues, a sufficient blood flow is not obtained. Accordingly, the anaerobic metabolism (glycolysis) is accelerated. As a result, the concentration of lactate is elevated. If the concentration of lactate is detected by using the spectroscopic imaging, then the difference of tissue metabolism can be observed and it becomes possible to distinguish vigorously proliferating cancer tissues from normal tissues and resting cancers. In cerebral ischemia, an increase of lactate is noticed even if the blood flow decreases slightly. Therefore, it is considered that the spectroscopic imaging makes it possible to accurately diagnose a region damaged by ischemia.
In general, however, the molecule which becomes the measurement object of the spectroscopic imaging is far lower in concentration than the water molecule. This results in a problem that the signal-to-noise ratio (SNR) is low. As compared with the ordinary MRI, the spatial resolution is made low and signal averaging is conducted with an ample measurement time. However, it cannot be said yet that a sufficient SNR is obtained. For example, in 1H spectroscopic imaging in the case of a typical 1.5 T MRI system, the number of voxels becomes approximately 32×32 with a spatial distribution of 1 cm cube and the SNR becomes approximately 0.1 with a measurement time of approximately 10 minutes. If smoothing processing of calculating the spatially moving averaging is conducted in order to increase the SNR, the spatial resolution falls extremely. If the spatial resolution is at least 1 cm, it becomes difficult to distinguish a small cancer tissue with high precision because of the partial volume effect. The “partial volume effect” means an effect that small changes of tissue contained in each voxel are decreased by influences of other most signals. In a diagnosis of conducting tissue classification with high precision, it is needed to improve the SNR without degrading the spatial resolution.
As for ordinary images, several edge-preserving filters have been proposed as the method for improving the SNR without degrading the spatial resolution. For example, as a well-known method, a median filter which selects a median from among spatial neighborhood (spatially neighboring voxels) in the object spatial dimensions can be mentioned. Besides, in JP-B-3472596 corresponding to U.S. Pat. No. 5,561,724, a method of detecting a dimension having a less concentration change from object spatial dimensions and conducting smoothing processing in the dimension alone is described.
In Kamiyama etc., “Method of small structure extraction from the ultrasound RF signal by using statistic analogy between echo signals,” The Japan Society of Ultrasonics in Medicine, Basic Technology study group paper, pp. 14-18, 2001, a method of reducing speckles in ultrasound imaging and conducting weighted smoothing is described. In this method, a higher weight is provided as the data value of an object voxel (also referred to as pixel) is similar to data values of neighboring voxels.
In JP-A-2004-129773 (corresponding to US 2004/0073112), a method of weighting and combining two images obtained by weighted smoothing which provides voxels having similar data values with higher weights and structure enhancing processing is described.
In JP-A-10-124664 (corresponding to U.S. Pat. No. 5,835,618), a filter which does hot lose a structure even if the dynamic range is conversely compressed is described.
In Bonny J M etc., “Multi-exponential analysis of magnitude MR images using a quantitative multispectral edge-preserving filter,” Journal of Magnetic Resonance, vol. 161, pp. 25-34 (2003), a method of utilizing an edge-preserving filter in order to measure the relaxation time using the MRI with high precision is proposed. For measuring the relaxation time, different measurements of the echo time are repeated and data having a plurality of signal values for each pixel are acquired. The relaxation time which is an attenuation factor is calculated from the decay of the signal value with the echo time. At this time, multiple values of each pixel are handled as multi-dimensional values, the multi-dimensional values for the object spatial dimensions are weighted according to similarity of the multi-dimensional value in the neighborhood and smoothed.
In JP-A-2001-8919, a method for improving the spatial resolution while preserving the SNR with respect to spectroscopic imaging data is described. Data are filled into a higher k-space by conducting linear prediction of data in a k-space dimension corresponding to the spatial dimension. As a result, it becomes possible to suppress Gibbs ringing as compared with the case where the higher k-space is filled with values 0 by zero-filling.